1. Field of the Invention
The present invention relates to a sheet feeding apparatus and, more specifically, to a technique for detecting state of sheet feeding on a paper feed path.
2. Description of the Background Art
As is well known, an image forming apparatus such as a printer or a copying machine is provided with a paper feeder for feeding sheets of paper one by one. If a mechanism for separating sheets of paper in the paper feeder does not work well, overlapped feeding occurs.
The term “overlapped feeding” means that a plurality of sheets are fed together, with a part of one sheet overlapped by a part of another sheet.
Japanese Patent Laying-Open No. 9-124173 (hereinafter referred to as “'173 application”) proposes a technique of mechanically preventing overlapped feeding by providing a peeling means that freely moves to be attached to/detached from one surface of a sheet, between a sheet feeding function and an image forming function.
Recently, however, various types of sheets having different colors, thicknesses and materials are used. Depending on the sheet type, sheet separation is impossible by the peeling function described in '173 application, and overlapped feeding may occur.
On the other hand, in an image forming apparatus, sometimes a feed-failure jam occurs, that is, a sheet of paper is not well fed but stopped on the feeding path. If such a feed-failure jam is left unaddressed, it possibly leads to a complicated jam as a paper path obstruction. Therefore, if a feed-failure jam is detected, paper feed should be stopped immediately.
As a typical method of detecting a feed-failure jam, a time from detection of a leading edge to detection of a trailing edge of a sheet at a certain position is measured, and if it exceeds a time corresponding to the sheet length, it is determined that a feed-failure jam has occurred.
According to this method of detecting a feed-failure jam, however, overlapped feeding would be mistaken as a feed-failure jam. If such erroneous detection of feed-failure jam occurs, paper feeding stops. Therefore, it becomes necessary for the user to manually remove the sheets fed in overlapped manner, which is rather troublesome.
Generally, when overlapped feeding occurs, it is considered easier for the user to have the sheets discharged by continuing paper feeding, than to stop feeding.
In view of the foregoing, Japanese Patent Laying-Open No. 2007-206572 (hereinafter referred to as “'572 application”) proposes a technique that alleviates burden of removing a sheet of paper, by reducing erroneous detection of feed-failure jam caused by overlapped feeding.
Specifically, on a paper feed path, a sheet passage sensor and a thickness sensor are arranged in this order from the upstream side to the downstream side in the direction of paper feeding. Time from detection of a leading edge to detection of a trailing edge of a sheet by the sheet passage sensor is detected, and whether or not feed-failure jam occurred is determined thereby. Specifically, if the detected time is longer than a normal time used for detecting feed-failure jam, it is determined that a feed-failure jam has occurred. Based on information from thickness sensor, whether or not overlapped feeding has occurred is determined. For instance, if the detected thickness is larger than the thickness of one sheet, it means that sheets are fed in an overlapped manner. If overlapped feeding is detected, paper feeding is continued. If feed-failure jam is detected but overlapped feeding is not detected, operation is controlled such that paper feeding is stopped.
The technique disclosed in '572 application uses a thickness sensor in addition to the sheet passage sensor, in order to reduce erroneous detection of feed-failure jam caused by occurrence of overlapped feeding. The technique, however, has the following problems.
The thickness sensor described in '572 application includes a light source for emitting light to the fed sheet of paper, and a function of measuring the intensity of light transmitted through the sheet that differs depending on the thickness of the sheet, and the sensor is adopted to detect sheet thickness based on the measured light intensity.
The types of paper, however, increase year after year as mentioned above and in order to attain detection accuracy of thickness sensor accordingly, necessary cost would be very high. This makes it difficult to adopt a thickness sensor for detecting overlapped feeding.
Meanwhile, it is among major problems recently in designing paper feeding system, to enable reliable detection of overlapped feeding and feed-failure jam by a combination of ON/OFF of sensors of one same type, that is, sheet passage sensors.